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Metamorphic Rocks
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Printed: February 11, 2015
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C HAPTER
Chapter 1. Metamorphic Rocks
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Metamorphic Rocks
Lesson Objectives
• Describe how metamorphic rocks are formed.
• Describe the properties of some common metamorphic rocks.
• Relate some common uses of metamorphic rocks.
Vocabulary
•
•
•
•
contact metamorphism
foliation
regional metamorphism
stable
Introduction
Metamorphism changes rocks by heat and pressure. These agents create an entirely new type of rock. Metamorphism
changes rocks physically and/or chemically.
Metamorphism
Metamorphic rocks start off as some kind of rock. The starting rock can be igneous, sedimentary or even another
metamorphic rock. Heat and/or pressure then change the rock’s physical or chemical makeup.
During metamorphism a rock may change chemically. Ions move and new minerals form. The new minerals are
more stable in the new environment. Extreme pressure may lead to physical changes like foliation. Foliation forms
as the rocks are squeezed. If pressure is exerted from one direction, the rock forms layers. This is foliation. If
pressure is exerted from all directions, the rock usually does not show foliation.
There are two main types of metamorphism:
1. Contact metamorphism results when magma contacts a rock, changing it by extreme heat ( Figure 1.1).
2. Regional metamorphism occurs over a wide area. Great masses of rock are exposed to pressure from rock and
sediment layers on top of it. The rock may also be compressed by other geological processes.
Metamorphism does not cause a rock to melt completely. It only causes the minerals to change by heat or pressure.
Hornfels is a rock with alternating bands of dark and light crystals. Hornfels is a good example of how minerals
rearrange themselves during metamorphism ( Figure 1.1). The minerals in hornfels separate by density. The result
is that the rock becomes banded. Gneiss forms by regional metamorphism from extremely high temperature and
pressure.
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FIGURE 1.1
(A) Hornfels is a rock that is created by
contact metamorphism. (B) Hornfels is
so hard that it can create peaks like the
Matterhorn.
FIGURE 1.2
(A) Regional metamorphic rocks often
display layering called foliation. (B) Regional metamorphism with high pressures
and low temperatures can result in blue
schist.
Uses of Metamorphic Rocks
Quartzite and marble are the most commonly used metamorphic rocks. They are frequently chosen for building
materials and artwork. Marble is used for statues and decorative items like vases ( Figure 1.3). Quartzite is very
hard and is often crushed and used in building railroad tracks. Schist and slate are sometimes used as building and
landscape materials.
FIGURE 1.3
(A) Marble is a beautiful rock that is commonly used for buildings. (B) Many of the
great statues of the Renaissance were
carved from marble. Michelangelo created this Moses between 1513 and 1515.
Lesson Summary
• Metamorphic rocks form when heat and pressure transform an existing rock into a new rock.
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Chapter 1. Metamorphic Rocks
• Contact metamorphism occurs when hot magma transforms rock that it contacts.
• Regional metamorphism transforms large areas of existing rocks under the tremendous heat and pressure
created by tectonic forces.
Lesson Review Questions
Recall
1. Why do the minerals in a rock sometimes rearrange themselves when exposed to heat or pressure?
2. List and describe the two main types of metamorphism.
Apply Concepts
3. How does layering form in metamorphic rocks?
4. What clues in metamorphic rocks tell you how they were formed?
Think Critically
5. Suppose a phyllite sample was exposed to even more heat and pressure. What metamorphic rock would form?
Points to Consider
• What type of plate boundary would produce the most intense metamorphism of rock?
• Do you think new minerals could form when an existing rock is metamorphosed?
For Table above,
• User:Jstuby/Wikipedia. http://commons.wikimedia.org/wiki/File:Lehigh_conglom.jpg . Public Domain.
• Mark A. Wilson (Department of Geology, The College of Wooster). http://commons.wikimedia.org/wiki/Fi
le:UpperTriassicYorkCountyPA.jpg . Public Domain.
• Courtesy of Ken Hammond, US Department of Agriculture. http://commons.wikimedia.org/wiki/File:USDA_Mineral_Sandstone_93c3955.jpg . Public Domain.
• Courtesy of United States Geological Survey/Mineral Information Institute. http://commons.wikimedia.org
/wiki/File:ShaleUSGOV.jpg . Public Domain.
• P K (Flickr:uair1). http://www.flickr.com/photos/56759497@N00/3870660087/ . CC BY 2.0.
• Flickr:oatsy40. http://www.flickr.com/photos/68089229@N06/9333525319/ . CC BY 2.0.
• User:ALM scientist/Wikipedia. http://en.wikipedia.org/wiki/File:Kehora_Salt_Cave.jpg . Public Domain.
References
1. (A) User:Fed/Ru.Wikipedia; (B) Flickr:CoreForce. (A) http://commons.wikimedia.org/wiki/File:Hornfels.
jpg; (B) http://commons.wikimedia.org/wiki/File:East_Face.jpg . (A) Public Domain; (B) CC BY 2.0
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2. (A) Image copyright katatonia82, 2013; (B) Renee French. (A) http://www.shutterstock.com; (B) CK-12 F
oundation . (A) Used under license from Shutterstock.com; (B) CC BY-NC 3.0
3. (A) Courtesy of the US Geological Survey and the Mineral Information Institute; (B) User:Prasenberg/Wikipedia.
(A) http://commons.wikimedia.org/wiki/File:MarbleUSGOV.jpg; (B) http://commons.wikimedia.org/wiki/Fi
le:Moses_San_Pietro_in_Vincoli.jpg . (A) Public Domain; (B) CC BY 2.0
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